1. Field of the Invention
The present invention relates to apparatus for protecting devices connected to AC power lines, telephone lines and/or coaxial transmission lines and, more particularly, to surge suppressors which protect those devices using a common ground.
2. Discussion of the Related Art
Today many homes use sensitive electronic devices as telecommuting and conducting business from one's home have become more commonplace. In addition to home entertainment centers with expensive audio/visual equipment, many homes now have personal computers, modems, printers, copiers, facsimile machines, telephone answering systems and home security systems. These sensitive electronic devices are connected to the outside world by means of telephone lines (both voice lines and high speed data lines), coaxial transmission lines (both cable TV and satellite dish antennas) and AC power lines.
Standards have been developed for residential cabling known as the EIA/TIA 570 standard and the Consumer Electronics Bus (CEBus.RTM.). These standards deal with Category 3 and Category 5 unshielded twisted pair cabling and coaxial cabling. These standards are described in "Cabling The Workplace '96" at pages 769 through 800.
Companies have recently begun offering complete home wiring systems which comply with the CEBus.RTM. standard. One example is the HomeStar.RTM. Wiring System offered by Lucent Technologies. According to Lucent Technologies literature, the HomeStar.RTM. System "[i]ntegrates a wide range of telecommunications and home automation technologies--from interactive home entertainment and personal communications to security and environmental management systems." The HomeStar.RTM. System does not, however, provide overvoltage protection for the different types of wires (RG6 coaxial cabling, Category 3 and Category 5 unshielded twisted pair cabling) used in the system.
Lightning is a major source of overvoltage conditions on residential wiring. The overvoltage condition can result from a direct lightning strike or it can be induced in the AC transmission lines by a nearby lightning strike. It is estimated that there are over 90 million lightning bolts striking the United States annually. Of course, only a small percentage strike buildings. However, each year thousands of homes and businesses are damaged or destroyed by lightning strikes. In 1990 residential claims for lightning damage exceeded one billion dollars. This number will increase as homeowners purchase more sophisticated electronics equipment.
Overvoltage conditions can also result from power line crosses caused, for example, by a vehicle striking a utility pole. Transients (voltage spikes) are caused by the utility company when it switches capacitor banks on and off line in order to correct the power factor (VI cos.theta.). Transients can also originate within the home when inductive loads such as electric motors are switched. Transients can also result from switching non-inductive loads and can be induced on wiring in the home.
Overvoltage conditions can also result from the loss of the AC neutral. If that occurs, devices that are normally connected to 110 to 120 volts AC are subjected to 220 to 240 volts AC. The loss of the AC neutral can result from corrosion of the meter pan at the service entrance where the meter is located outside the building and is exposed to the elements. Most plug-in secondary surge suppressors being sold today are not designed to withstand the loss of the AC neutral and may create a fire hazard when the AC neutral is lost.
Primary overvoltage protection for telephone lines is provided by surge arresters located in network interface devices mounted on the outside of the home. See devices 73 in FIG. 3 of U.S. Pat. No. 4,979,209 issued to Collins et al on Dec. 19, 1990. Grounding for these overvoltage protection devices is provided by means of an earth ground brought into the enclosure at the time of installation and attached to ground bus 71 at terminal 71A.
Coaxial transmission lines carrying cable television signals may be brought into a home through network interface devices mounted on the outside of the home. See U.S. Pat. No. 3,394,466 issued to Schneider et al on Feb. 28, 1995. As shown in FIG. 1 of that patent, the coaxial cable is grounded by connecting a ground strap 228 from module 220 to ground bus 71 which is then connected to earth ground. See column 4, lines 50-54. Coaxial surge arresters for protecting coaxial transmission lines from overvoltage conditions are also known. See U.S. Pat. No. 4,616,155 issued to Guichard on Oct. 7, 1986 and U.S. Pat. No. 5,566,056 issued to Chaudhry on Oct. 15, 1996.
Many homeowners attempt to protect their valuable electronic equipment with plug-in surge suppressors. Such devices do not protect equipment from the large amplitude pulses caused by lightning strikes, although they do offer some protection against low energy transients originating within the home. Moreover, plug-in surge suppressors are generally located far from the earth ground where the electric utilities are brought into the building. A surge suppressor is only as good as the ground to which it is connected. A plug-in surge suppressor which protects both AC power lines and telephone lines is shown in U.S. Pat. No. 4,438,477 issued to Cawley on Mar. 20, 1984.
Plug-in surge suppressors sold prior to 1998 had to meet the requirements of Underwriters Laboratories Standard 1449, First Edition ("UL 1449 Ed 1") dated Aug. 28, 1985. On Aug. 15, 1996 UL 1449 Second Edition ("UL 1449 Ed 2") was released and became effective Feb. 16, 1998. It requires new tests that were not required by UL 1449 Ed 1. For example, a new test that must be passed to qualify under UL 1449 Ed 2 is that a plug-in surge suppressor must not create a fire hazard in the event the AC neutral is lost. Since UL 1449 Ed 2 was promulgated in August 1996, companies have been struggling to comply with the new test requirements.
In addition to the need for plug-in surge suppressors that will meet UL 1449 Ed 2, there is also a need for surge suppressors that will meet UL 1449 Ed 2 and that can be installed in electronic devices such as TVs and VCRs at the time of manufacture to protect those devices from overvoltage conditions appearing on AC power lines and coaxial transmission lines.